Isotope geochemistry of oils from fields and mud volcanoes in the SouthCaspian Basin, Azerbaijan

Ibragim S. Guliyev, Akper A. Feizulayev and Dadash A. HuseynovHusf!YnJavid, 29A, 370143, Institute of Geology,Baku, Azerbafjan

ABSTRACT: This study is based on the isotope-geochemical characteristics of oilsfrom 53 fields in the South Caspian Basin - from Upper Cretaceous, Eocene,Oligocene-Lower Miocene (Maykop), Middle and the Upper Miocene (Diatom) andPliocene. The isotopic composition of carbon (813q allows two main groups of oilsto be identified: (1) isotopically heavy and (2) isotopically light oils. The first groupincludes oils assumed to be generated by Upper Cretaceous, Eocene and Oligocene-Lower Miocene sources. The second group consists of oils assumed to be generatedby Diatom (Middle and Upper Miocene) sources. Isotope-geochemical and biomar-ker parameters demonstrated that oils in the Pliocene reservoirs are not syngeneticto their enclosing deposits. The isotopic composition of carbon in oils andhydrocarbon gases in mud volcanoes and fields and the biomarker parameters of oilsallow several isolated stratigraphic oil- and gas-producing complexes (source rocks)in the Mesozoic, Palaeogene-Lower Miocene and Middle-Upper Miocene to bepostulated. Oils in the Pliocene reservoirs and in mud volcanoes consist of mixed

oils generated by the Pre-Diatom and Diatom deposits and of oils from only one ofthe above-mentioned complexes.

KEYWORDS: carbon isotope ratios, source rock, reservoir, hydrocarbon seep, mud volcano

INTRODUCTION

The oil fields in the South Caspian Basin (SCB) have beendeveloped for over 150 years. The total production is about1.4 x 109 tons of oil. Despite the long history of production andthe study of the oil fields, the source rocks for the oil areunknown. The hydrocarbon finds in the SCB occur in reser-voirs over a wide stratigraphic range, from Upper Cretaceous toPliocene-Anthropogene. The oil fields mostly occur in theLower Pliocene (productive Series, PS), which contains up to90% of the discovered reserves. The occurrence of petroleumin reservoirs over a wide age range and the extreme thickness ofthe sedimentary fill «25 km) of the SCB have no analogues inthe world. It is possible that there exist several oil-generatingcomplexes (petroleum systems) (Ali-zade et ai. 1975). It isdifficult to evaluate the source rocks which have contributed to

the formation of the hydrocarbon accumulations in the Pro-ductive Series. This makes it difficult to assess the oil and gaspotential both in the Productive Series and in the deeper andshallower reservoirs. This article presents results from isotope-geochemical correlation of oil samples from 53 oil fields inreservoirs from the Upper Cretaceous to the Upper Pliocene.These new data make it possible to evaluate the oil and gaspotential of the South Caspian Basin.

GEOLOGICAL SETTING

The South Caspian Basin is adjacent to the orogenic zones of theCaucasus, Kopet-dag and Elburz. The basin contains into anumber of intermontane depressions and troughs of differingtectonic structure: the Kura intermontane depression consistingof the Upper, Middle, and the Lower Kura depressions and theSouth Caspian Basin, containing the Shamakha-Gobustan and

PetroleumGeoscience,Vo!. 72001, pp. 201-209

the Absheron depressions. The north and the northeast marginof the South Caspian Basin is formed by the fold mountainsystem of the Greater Caucasus and its underwater continuation,represented by the Absheron-Pre-Balkhan zone of anticlinalrises (the Absheron archipelago). The southern. margin is formedby the fold mountain system of the Lesser Caucasus (Fig. 1). Inthe east the SCB is bounded by the Dzirul massif.

The crystalline basement of the SCB varies greatly in depth andhas a tendency to subside in steps. In the west it occurs at depths of4-6 km, in the Middle Kura depression at 16 km, in the LowerKura it subsides to a depth of 20 km and in the South CaspianDepression it is at a depth of 21-25 km (Fig. 2). The marginsbetween the depressions are the buried rises of the pre-Alpinebasement and the deep faults that limit them. In the depressionsthere are a number of oil- and gas-bearing regions (OGBR) (Fig. 1).In the Middle Kura depression one can distinguish the Evlakh-Agjabedi OGBR, the Ganja OGBR and the OGBR of the inter-fluve of the Kura and Gabyrry. The Lower Kura, .Shamakha-

Gobustan and Absheron depressions correspond to the oil and gasregions of the same names. In the South Caspian Depression onecan distinguish the OGBR of Baku and the Abshewn archipelago.

In the OGBR of the Middle Kura the hydrocarbon-bearingreservoirs comprise: volcanogenic and calcareous deposits inthe Upper Cretaceous and terrigenous and carbonate rocks inthe Eocene, in the Maykop series (Oligocene-Lower Miocene)and in the Chokrak (lower Middle Miocene) co.mplexes (Fig. 3).In the Lower Kura depression hydrocarbon-bearing reservoirsoccur in: sandy deposits' in the Lower and Upper Pliocene;deeper strata have not yet been drilled here.

In the Shamakha-Gobustan trough the reservoirs span alarge stratigraphic range: from the calcareous rocks of theUpper Cretaceous in the north and northwest, to sandy-clayey

1354-0793/01/$15.00 @ 2001 EAGE/Geological Society of London

202 I S. Guiiyevet al.

~ anticline structure, oilfield

~ gas field, oil and gas field

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strata of the Lower Pliocene in the southeast. In the Absheron

trough (peninsula) oil and gas are confined to Middle-UpperMiocene (Diatomic) and Lower Pliocene strata. At Baku and

the Absheron archipelago the oil and gas occurs in sandyreservoirs of the Lower Pliocene.

OIL SAMPLES STUDIED

Oils were studied from 53 fields in the Absheron,

Evlakh-Agjabedi, Shamakha-Gobustan and Lower Kura oil

Fig. 2. Schematic structural map onthe surface of the basement in theSouth Caspian Basin. 1,2: isohypses onthe surface of the basement (km) - 1,on geophysical data, 2, on geologicaldata; 3: crusta! fault zones; 4: zone ofabsence of granitic layer.

and gas regions in the Baku and Absheron archipelagos and inthe interfluve of the rivers Kura and Gabyrry, comprisingreservoirs from Upper Cretaceous to Upper Apsheron levels.A list of the oil fields and the studied samples is given inTable 1. The isotope composition of oils from 20 natural oilseepages associated with mud volcanoes in the Absheron,Shamakha-Gobustan and Lower Kura regions was studied aswell. The studied mud volcanoes are listed in Table 2. The

locations of the studied seepages (mud volcanoes) are shown inFigure 4.

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METHODOLOGY

The most difficult task in evaluating oil source rocks is the

determination of direct genetic associations between oils present in

reservoirs and specific oil source rocks, The determination of these

associations will help solve a number of important problems, e,g,

the origin of the oil, analysis of conditions and facies favourable foroil generation, assessment of hydrocarbon reserves and potential of

oil and gas basin, The task can be studied in two ways, The first is

the comparative analysis of some compounds and their correlationin oils and in the organic matter of rocks, e,g, normal alkanes,

isoprenoids, steranes, triterpanes, some groups of aromatic hydro-

carbons etc, The second approach is based on the study of the

isotopic composition of the matter, principally the isotopic com-

position of carbon, We prefer the isotope method of the study of

the organic matter and oil, for it is not limited to certain com-pounds and structures but derives from the characteristics of the

chemical element which makes up the basic mass of the matter,

Moreover, isotope relations are less exposed to changes deter-mined by secondary alteration. This article presents results of

studies of the stable isotopes of the total carbon of oils and the

carbon of the alkane and aromatic fractions on 152 samples (wells),

203

b.

Fig, 3. Schematic stratigraphic columnfor (a) the western South Caspian Basinand (b) Pliocene division for Absheron,Shamakha-Gobustan, Lower Kura areasand SW Caspian shelf,

On the basis of the results of isotope analyses we constructed

and interpreted graphs and histograms of frequency of distri-

bution of values of isotope ratios for oils from the Upper

Cretaceous-Upper Pliocene reservoirs, for the oils from reser-

voirs of a particular age and also for the oils of each oil and gas

region, The results of isotope analyses of oils from naturalseepages were interpreted by the same method.

Studies of the organic matter and oils using molecular fossils(biomarkers) allowed us to conduct oil-oil and oil-source rock

correlations and to determine the stratigraphic age of the oil as

well as the maturity of oils related to the level of the catagenetic

transformation of the producing kerogen,

The following parameters were used: level of isomerization

of hopanes, steranes, sterane aromatization, correlation of the

aromatic steroids ete. We used the following highly informative

and widely applied biomarker parameters: degree of isomeriz-

ation of sterane [aa C29 (20S/S+ R)] and aromatization ofmono aromatic sterane [C28 triaromatic sterane/C28 triaromatic+C29 monoaromatic sterane],

Determination of the maturity of hydrocarbon gases is based

on the calculations of the dependence between the isotope

composition of carbon in gases and their catogene maturity

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Age Stage Lithology voir

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Table 1. South Caspian Basin: studied oilfield samples Table 1. Continued

Depth Depth

Number on Oil Well top-bottom Number on Oil Well top-bottom

Figure 1 field number (m) Reservoir Figure 1 field number (m) Reservoir

Absheron peninsula 26 Gum 348 2656-2682 Productive Series

1 Puta 2128 482-500 Productive Series 27 Bahar 148 4500--4572 Productive Series

Puta 805 728-801 Productive Series Bahar 182 4660--4700 Productive Series

Puta 1162 858-878 Productive Series Bahar 3 Productive Series

2 Bibieibat 2565 1504--1578 Productive SeriesBaku archipelago

Bibieibat 3818 1883-1886 Productive Series

Bibieibat 2566 1508-1564 Productive Series28 Duvanny 320 3548-3614 Productive Series

Bibieibat 2681 Productive Series29 Sangachal 551 Diatom

3 Zykh 314 2219-2278 Productive Series Sangachl 9 Diatom

4 Buzovna 1057 1820-1902 Productive Series30 Garasu 122 Productive Series

Buzovna 1324 1942-1957 Productive Series Shamakha-Gobustan OGBR

Buzovna 1324 2013-2030 Productive Series 31 Astrakhanka Up. Cretaceous

5 Gala 1300 1975-1987 Productive Series 32 Adjiveli 12 Maykop

Gala 1494 2118-2194 Productive Series 33 Umbaki 135 682-710 Chokrak

6 Gousany 1701 3537-3562 Productive Series Umbaki 308 308-327 Chokrak

7 Ramany 3312 1417-1446 Productive Seties Umbaki 101 1334--1380 Chokrak

Ramany 3683 1919-1935 Productive Series Umbaki 34 424--482 Chokrak

Ramany 3805 2315-2323 Productive Series Umbaki 114 960-990 Maykop

8 Surakhany 945 2574--2590 Productive Series Umbaki 196 1567-1648 Maykop9 Garachukur 64 2418-2444 Productive Series Umbaki 11 1314--1396 Maykop

Garachukur 927 2510-2542 Productive Series Umbaki 21 1055-1121 Maykop

10 Binagady 1167 615-638 Productive Series 34 Dashgil 15 2629-2630 Productive Series

Binagady 1448 665-787 Productive Series 35 Kanizadag 29 417 5--4200 Productive Series

Binagady 2464 762-801 Productive Series Kanizadag 25 2398-2532 Productive Series

Binagady 2464 853-910 Productive Series Lower Kura OGBRBinagady 2759 626-654 Productive Series 36 Pirsagat 204 1740-1747 Productive Series

11 Sulutepe 2696 Productive SeriesPirsagat 85 4110--4130 Productive Series

Sulutepe 2597 1199-1201 Productive SeriesPirsagat 111 Productive Series

Sulutepe 2599 1190-1201 Productive Series 37 Kalamadvn 161 1380-1400 Productive Series12 Kushkhana 1862 352--414 Productive Series

Kalamadyn 72 1728-1783 Productive Series13 Masazyr 105 90-204 Productive Series

Kalamadyn 24 1674--1693 Productive SeriesMasazyr 104 122-258 Productive Series

Kalamadyn 126 1619-1637 Productive SeriesMasazyr 111 82-290 Productive Series 38 Kichik Kharami 3 1900-2000 Productive Series

14 Balakhany 1016 312-358 Productive Series 39 Kurovdag 958 843-852 Upper PlioceneBalakhany 21 036 241-295 Productive Series

Kurovdag 1008 258-263 Upper PlioceneBalakhany 1107 240-266 Productive Series

Kurovdag 918 1118-1242 Upper PlioceneBalakhany 3548 272-295 Productive Series

Kurovdag 127 1936-1955 Upper PlioceneBalakhany 3178 728-759 Productive Series

Kurovdag 148 2238-2278 Upper PlioceneBalakhany 3230 841-879 Productive Series

Kurovdag 934 2020-2030 Productive Series15 Sabunchi 3728 1358-1374 Productive Series

Kurovdag 517 2511-2556 Productive SeriesSabunchi 2345 1066-1114 Productive Series

Kurovdag 889 2789-2790 Productive Series16 Kergez 263 1661-1759 Productive Series

Kurovdag 1263 2919-2936 Productive Series17 Guzdek 210 2024--2060 Productive Series 40 Kursanga 401 4119-4135 Productive Series18 Garadag 511 3100 Diatom

Kursanga 4 4884--4924 Productive SeriesGaradag 352 2606-2640 Diatom

Kursanga 298 Productive SeriesGaradag 508 2812-2906 Diatom 41 Garabagly 144 2784--2797 Productive Series

Garadag 1 Productive Series 42 Hilly 15 1665-1728 Productive SeriesGaradag 73 1945-2003 Productive Series

Hilly 418 1128-1180 Upper Pliocene

Absheron archipelago 43 Neftchala 1083 1170-1256 Producti ve Series

19 Gunashli 170 3144--3188 Productive Series Neftcha!a 709 1821-1956 Productive Series

Gunashli 182 2835-2891 Productive Series Neftchala 3001 3249-3254 Productive Series

Gunashli 2 2994--3039 Productive Series 44 Mishovdag 276 1176-1203 Upper Pliocene

Gunashli 257 2690-2751 Productive Series 45 Kalmas 416 Productive Series

Gunashli 118 2749-2884 Productive SeriesOGBR of the interfluve of the Kura and Gabyrry

Gunashli 81 2913-2930 Productive Series 46 Demir Tepe 7 4200 EoceneGunashli 260 2705-27 053 Productive Series 47 West Gurzundag 1 4230--4450 EoceneGunashli 189 2329-2368 Productive Series 48 Palantekan 2 5112-5129 EoceneGunashli 16 3310-3430 Productive Series 49 T arsdallar 1 EoceneGunashli 124 3004--3068 Productive Series T arsdallar 24 4236--4256 EoceneGunashli 258 3030-3050 Productive SeriesGunashli 122 Productive Series Evlakh-Agjabedi OGBR

20 Azeri 1 2447-2489 Productive Series 50 Zardob 7 4093--4175 Eocene

21 Pirallahi 654 570-655 Productive Series 51 Shaftakhal 250 4200 Eocene

22 Absheron kupesi 4 Productive Series 52 Muradhanly 37 4135--4189 Up. Cretaceous

23 Kapaz 3 3628-3682 Productive Series Muradhanly 247 Eocene

24 Janub 57 2981-2987 Productive Series Muradhanly 232 4168 Maykop

25 Neft Dashlary 2001 385-404 Productive Series Muradhanly 246 4110--4140 Maykop

Neft Dashlary 3 383-389 Productive Series 53 Djafarlv 21 4030 Eocene

Djafarly 28 Eocene

Isotope geochemistry of oils in the South Caspian Basin, Azerbaf;cln

~cvcchi

0""

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) ( \il\\\

SEA

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Fig. 4. Location map of mud volcanoes. 1, anticline structure; 2,mud volcanoes (black circles are studied mud volcano seep); 3,boundary between OGBR.

(Faber 1987). On the basis of this dependence and data onvitrinite reflectivity (Ro) in the studied region, one can deter-mine the hypsometric depth of location of the gas source andof the gas from a specific mud volcano. Information about thedeep structure of the region where a volcano is located allowsus to determine the stratigraphic depth of its root.

To determine the hypsometric and stratigraphic depths ofthe source of gases from the mud volcanoes we used the

205

relationships between the isotope composition of carbon (ICC)of ethane and Ro (Faber 1987) which is as follows:

b13C(C2H6)(%0) = 22.6 19 Ro(%) - 32.2. (1)

Application of the relationship between the ICC of methaneand Ro was not feasible because of a possible isotopic mixing ofthermocatalytic and biochemical methane.

Carbon isotopes were examined by combustion by sealedtube and moving flow methods, measured using a VG 602Double Collector and CJS Sigma Triple Collector StableIsotope Ratio Mass Spectrometers. Data are reported relative toPDB using the NBS 22 secondary standard with value of- 29.81 %0.

GC-MS analyses of oils were conducted using a HewlettPackard 5890 gas chromatograph and a Finnigan 4000 massspectrometer. A fused silica capillary column (30 m x 0.25 mm)coated with 0.25 J..lm DB5-MS was temperature programmedfor: Alkanes - 50°C for 1 minute, 50°C to 180°C at 35°C perminute, 180°C to 300°C at 1.7°C per minute, isothermal at300°C for 25 minutes; Aromatics - 80°C for 1 minute, 80°C to300°C at 4°C per minute, isothermal at 300°C for 24 minutes.

Helium was used as the carrier gas. The ionization voltage was70 V and the scan time was 1 s.

RESULTS OF ISOTOPE STUDY

Reservoired oils

The isotopic values of hydrocarbon 813C in the oils of the SCBvary widely, from - 28.0%0to - 24.34%0 for the oils and from- 29.1 %0 to - 24.8%0 for the alkane fraction. The oils in the

SCB can be grouped into two classes: (1) isotopically light, with813C values of - 28.0%0 to - 27.0%0 for the total carbon and

- 29.1 %0 to - 27.0%0 for the carbon of the alkane fraction;and (2) isotopically heavy, with values of - 26.5%0 to - 24.0%0and - 26.5%0 to - 24.5%0 for the total carbon and the alkane

fractions, respectively (Fig. 5). Mostly the oils in the SCB arerepresented by oils of the second group (58-69'10 of the exam-ined samples), whereas the isotopically light oils make up31-42% of the samples. An important observation is that thereis a distinct, regular change in the isotopic values through thestratigraphic section. Thus, the isotopically lightest oils aretypical for the Upper Cretaceous reservoirs, which have valuescorresponding to 813C in alkane fraction and in the whole oilcarbon of - 28.15%0 (- 28.0%0). Isotopically light oils occurin these reservoirs: Eocene - 28.32%0 (- 27.86%0); Maykop(Oligocene-Lower Miocene) - 28.05%0 (- 27.64%0) andChokrak (Lower-Middle Miocene) - 27.95 (- 27.52%0). Iso-topically heavy oils occur in these reservoirs: Diatom suite(Middle-Upper Miocene) - 26.45%0 (- 26.13%0) and Plioceneage - 26.36%0 (- 25.75%0) (Figs 5, 6). At the same time therange between the upper and lower limits of the 813C valuesincreases in the same direction. Oils in Pliocene reservoirs are

distinguished by the largest variation in isotope values: 22.6°;(Jofthe alkane fraction are isotopically light and 9.68% isotopicallyheavy, whilst the main mass (67.7%) are of intermediate value.

Potential source rocks

All of the studied oils appear to derive from source rocksformed in nearshore-marine and delta conditions. This is

indicated by the 813Calk versus 813Carom diagram (Fig. 7), wherethe oils plot in the field of marine organic matter and arepositioned along the border line separating continental andmarine organic matter types. This suggestion is also supportedby the Pr/Ph ratio, only a few of the oils fall in the 1.58-2.12range, and by sulphur contents not exceeding 0.4%. These

Table 2. South CasPianBaJin: thestudiedoil andgaJ JeepJrelatedto the mud ,;o!canoes

Number on Mud Type of Location

Figure 4 volcano studied sample area

I Cheildag Oil Shamakha-Gobustan2 Airantekan Oil, gas Shamakha-Gobustan3 Bahar Oil, gas Shamakha-Gobustan4 Matrasa Oil Shamakha-Gobustan5 Demirchi Oil Shamakha-Gobustan6 Melikchobanly Oil, gas Shamakha-Gobustan7 Djengi Oil Shamakha-Gobustan8 Kjrkishlag Oil Shamakha-Gobustan9 Kirdag Oil Shamakha-Gobustan

18 Kyrlykh-Enikend Oil, gas Shamakha-Gobustan16 Perekishkul Gas Shamakha-Gobustan20 Shikhzagirly Gas Shamakha-Gobustan10 Charagan Oil, gas Shamakha-Gobustan11 Umbaki Oil Shamakha-Gobustan12 Shorbulag Oil, gas Absheron13 Kirmaki Oil Absheron14 Zigilpiri Oil Absheron15 Kyrlykh lake Oil Lower Kura17 Kyrlykh-Kharami Oil Lower Kura19 Akhtarma Pashaly Oil Lower Kura

206 I S. Guljyevet al.

All reservoirs

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results suggest that the initial organic matter had a mixedcontinental-marine composition, with a predominant sapro-pelic input (Abrams & Narimanov 1997; Guliev et ai. 1997).These conclusions are confirmed by the Cn:Cz8:CZ9 normalsteranes and isosteranes correlation (about 33%:35%:32% and31%:36%:33%, respectively). The relatively high values of theoleanane index suggest a high input of continental organic

Pliocene average: -26.36range: (-27.7 to -24.8)

Diatom

upperCretaceous

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Fig. 6. Average values and ranges of carbon isotope ratios in thealkane fraction of oils from different age reservoirs in the SouthCaspian Basin.

matter into the palaeobasin (Inan et al. 1997), whilst themoderate values of the gammacerane index indicate a salineenvironment in the palaeobasin in the area of organic matteraccumulation and fossilization. The obtained geochemical dataprove that the oils present in Pliocene reservoirs are notsyngenetic to the deposits which contain them, as it is knownthe Pliocene basin was a closed freshwater basin (Kerimov et al.

1991; Lerche et ai. 1997; Reynolds et a!. 1998), with intensiveinput of continental organic debris carried together withterrigenous clastic material. If the Pliocene oils were generatedfrom source rocks deposited in such conditions, the geochemi-cal composition would be different, with for example a highPr/Ph ratio (> 3), a predominance of sterane CZ9 above steraneCn and CZ8' an absent or very low gammacerane index, a veryhigh oleanane index, ete.

The occurrence in the Pliocene reservoirs of both isotopicallylight and isotopically heavy oils, with differences in ol3C valuesfrom 2.1 %0 to 3.29%0, suggest they have been generated from atleast two different source rocks (Peters & Moldowan 1993;Chung et al. 1992), for example the pre-Diatom (Cretaceous-Lower Miocene) and the Diatom (Middle-Upper Miocene). Thepresence of both mixed oils, produced from the Palaeogene-Lower Miocene and Diatom sources and of oils derived from a

single source is characteristic of the Pliocene reservoirs.

Oil seeps

Isotopic studies of oil seeps associated with mud volcanoes hasallowed two oil groups to be differentiated: oils with a typicalPalaeogene-Lower Miocene carbon isotopic signature and thoserepresenting mixed oils generated from both Palaeogene-LowerMiocene and Diatom sources. Figure 8 illustrates the correlationof mud volcano seeps according to the isotopic composition ofcarbon in the saturated fraction. It is inferred from the figure thatabout 50% of the mud volcanoes release Palaeogene-LowerMiocene sourced oils. Around 17% of the mud volcanoes largelyrelease oils sourced from the Diatom complex and 33% of themrelease a mixture having approximately equal share of oils fromthe Palaeogene-Lower Miocene and Diatom sources.

Carbon isotope composition and maturity level ofhydrocarbons

The maturity of the oils in the mud volcano seeps, expressedas the equivalent vitrinite reflectance (Ro) calculated from the

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Isotope geochemistry of oils in the South Caspian Basin, Azerbaijan

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Upper Cretaceous reservoirs* Mudvolcanoes

sterane aromatization level (C2striaromaticjC2striaromatic+Czsmonoaromatic), indicates a low maturity level (Ro=0.46-0.64%). The application of other maturity parameters, like ho-pane and sterane isomerization ratios, etc. is not possible due tothe very high biodegradation and oxidation of the oils. It shouldbe noted that such low maturity is typical for the oils of the fieldsin the studied region and the Caspian Sea. Those oils very rarelyreach the value of Ro=0.68%. As rule ~) is 0.61 % (Fig. 9).

For gases, quantitative calculations can be based on therelationship of the carbon isotopic composition of the hydro-carbon gases to the level of their catagenetic maturity (Equation(1)). Such calculations show that ethane from eight mudvolcanoes (Charagan, Shorbulag, Shikhzagirly, Perekushkul,Kyrlykh-Enikend, Melikchobanly, Airantekan and Bahar),located in different parts of the northwestern and northernBanks of the South Caspian Basin (Fig. 4), has ~ values of1.3-1.79%. Vitrinite reBectance values in the SCB have been

measured down to depths of 5300 m and extrapolated to thedeep-seated horizons. On this basis, the ethane of the studiedmud volcanoes in the region is likely to have been generatedat depths of 7-8 km. These depths in the northern and inthe northwestern Banks of the depression correspond toJurassic-Cretaceous strata. In the central, deeper, part ofthe basin the ethane formation interval is located at depthscorresponding to Palaeogene-Miocene strata.

83C,%0

-22.00

Fig. 7. Cross-plot of carbon isotopecompositions in alkane and aromaticfractions of reservoired and mudvolcano oils in the South CaspianBasin.

The information oudined suggests the existence of severalstratigraphically isolated source rocks in Mesozoic, Palaeogene-Lower Miocene and Diatom intervals. Thus, the oil and gasformation 'kitchens' occur at different depths and differentstratigraphic levels. According to the vertical zonation schemeof oil and gas formation (Fig. 9), all of the oils in the knownfields and mud volcanoes correspond to the early stage of theoil window. The gases in the fields and mud volcanoescorrespond to the zone of wet gas and methane formation.

Relative contributions of the source rocks

The definition of the limiting 013C values which characterizethe pre-Diatom and Diatom oils, allows us to estimate the realcontribution of each oil-generating complex (source rocks) insupplying the oils in the Pliocene reservoirs.

Based on the isotopic composition of the oils, Palaeogene-Lower Miocene and Diatom source rocks appear to havecontributed approximately equally to the oils in the Pliocenereservoirs of the Absheron peninsula. The same is true for theoils in the Pliocene reservoir of the Shamakha-Gobustan

OGBR and Baku archipelago, although there are slighdy moreoils sourced from the Diatom complex. Approximately three-quarters of oils in the Lower Kura OGBR are formed fromPalaeogene-Lower Miocene source rocks. The source rock

208 L S. Gulryev et al.

All reservoirs

24.7%

Oils from mud volcano seeps

29.17%

Fig. 8. Frequency distribution of carbon isotope ratios of the alkanefraction of mud volcano oils, compared with values from allreservoirs (Fig. 5).

Amount-Ro.%

0.4

Maturity degreeof the oils in the

South Caspian depression

0.5

0.6

0.7

0.80.9

1.0

1.31.4Maturity degree

of the gases in theSouth Caspian depression

2.0

Stages of oil generation

0 immature oil mea~y ~ peak nu lateandcondensatelWetgas 8 methane

Fig. 9. Schematic plot of maturity (in R,,) against intensity ofhydrocarbon generation showing vertical zonality of hydrocarbonformation in the SCB.

for two-thirds of the oils in the Pliocene reservoirs of the

Absheron archipelago is the Diatom suite (Fig. 10).The general trend is that from land to sea the oils are

isotopically heavier (Fig. 11). Such regularity can be explained

Lower Kura OGBR

3530.77

30

25

~

}20

1'510

Absheron peninsula (OGBR)

45 42.85

40

35

30'"

f251'0

151429

10

Baku and Absheron archipelago

50

45

47.36

40

35

~30~25

!20

15

10

Carbon isotope ratios, "00

Fig. 10. Frequency distribution of carbon isotope ratios of thealkane fraction of oils from different OGBR in the South CaspianBasin.

by the 'oil window' occurring in younger strata. Offshore thereis a considerable increase in thickness of young Pliocene-Quaternary strata and underlying rocks are buried to largedepths. However, according to the B13C values of methane ofmud volcanoes the inverse tendency takes place (Dadashev et al.1986; Guliev & Feizullaev 1996).

Based on isotopic character, the oils in the Pliocene reser-voirs of the SCB consist on average of equal inputs fromPalaeogene-Lower Miocene and Diatom sources. Taking into

25

20'"

15 +u.

I10

5

0

"' 0 "' 0 "' 0 "' 0 "' 0<Xi <Xi ,..: ,..: <Xi <Xi .,; .,; .;"i "i "i "i "i "i "i "i N

.:, ,;, .:, ,;, .:, ,;, .:, ,;, .:, "',..: ,..: <Xi

Caon isote ratios')'>l"

0 0 0 0 0 0"N N, , ,'" 5' '" 5' - - - 5' '" 00

.,; ro .,;1£

N

Cart;on isote "'.05":"'00

0 0 '" 0 0 0 0gj..c-

N..c- ..c-

, , ,0 '" 5' - - - 5' '" 5' '"0

.,; .,; ro

Ca;;'n isot.;j;e "'.05:"00

5' '" 5' '" 5' '" 0 0 0.,; <0 <0 '" '"

0 '" 5' 0 0 0..N

Isotopegeochemistryof oils in the South CaspianBasin, Azerbaijan 209

..

Okm 55 km

Fig. 11. Map of the distribution of carbon isotope ratios in oils in the Pliocene reservoirs.

consideration the fact that the thickness of the Oligocene-Lower Miocene source rocks in this part of basin increases totwice the thickness of the source rocks of the Diatom suite, we

suggest that only part of the hydrocarbons generated from thepre-Diatom source is present in the Pliocene reservoirs. Weconclude that 50% of the hydrocarbons generated fromPalaeogene-Lower Miocene sources remain unrecognized.

CONCLUSIONS

. Two main groups of oils exist in the South Caspian Basin:isotopically light and isotopically heavy. The isotopicallylight oils were generated from Palaeogene-Lower Miocenesource rocks and the isotopically heavy oils mainly fromDiatomic source rocks.

. Isotope-geochemicaland biomarker parameters demon-strated that oils in the Pliocene reservoirs are not syngeneticto their enclosing deposits.

. The oils in the Pliocene reservoirs are

separate source rocks. The role of eachin supplying oils to the various reservoirsvanes.

. The carbon isotope composition of oils and hydrocarbongases in mud volcanoes and fields and biomarker parametersof oils suggest the existence of several isolated stratigraphicoil- and gas-producing complexes (source rocks) in theMesozoic, Palaeogene-Lower Miocene and Middle-UpperMiocene deposits.

. This isotopic research into the sources of the hydrocarbonstestifies to the high prospectivity of the South Caspian Basinand the likelihood of new oil and gas accumulations beingfound.

mixtures fromof the sourcesand sub-basins

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Lerche, 1., Ali-Zade, A. K., Guliev, 1., Bagirov, K, Nadirov, R., Tagiyev, M.& Feizullaev, A. 1997. Soutb CasPianBasin: Stratigraphy,Geocbemistryand RiskAnalysis. Nafta-Press, Baku.

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Received 21 June 1999; revised typescript accepted 8 September 2000